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508.1. First, the active player declares attackers. This turn-based action doesn't use the stack. To declare attackers, the active player
follows the steps below, in order. If at any point during the
declaration of attackers, the active player is unable to comply with
any of the steps listed below, the declaration is illegal; the game
returns to the moment before the declaration (see rule 717, "Handling
Illegal Actions").

508.1a The active player chooses which creatures that he or she controls, if any, will attack. The chosen creatures must be untapped,
and each one must either have haste or have been controlled by the
active player continuously since the turn began.

508.1b If the defending player controls any planeswalkers, or the game allows the active player to attack multiple other players, the active
player announces which player or planeswalker each of the chosen
creatures is attacking.